The invention relates to a method for generating ultrapure oxygen bearing steam by oxidation of hydrogen in a closed torch with separate feed lines for hydrogen and oxygen and a discharge line for discharging the steam which is formed to a process chamber. The invention also relates to an apparatus for carrying out the method.
Oxidation processes are carried out in a wide range of process steps involved in the processing of semiconductor wafers for the fabrication of semiconductor components. In this context, it has proven expedient to use steam instead of pure oxygen as the oxidizing agent. However, this steam which is used for oxidation purposes cannot be produced by simply evaporating water, but rather, on account of the extremely high purity requirements, has to be produced by combustion of hydrogen. For this purpose, oxygen and hydrogen have to be provided as ultrapure gases and burned in a special torch. A torch of this type is also known as a balloon.
On one side, a balloon of this type includes an injector for the ultrapure gases which are to be supplied and which are fed to the injector via concentric passages and, on the opposite side, a discharge line is provided for discharging the steam, which is formed during the combustion, to a process chamber. The quantities of hydrogen and oxygen supplied are controlled with the aid of mass flow controllers.
To prevent any contamination, the entire balloon, including the injector and the feed and discharge lines, are made from quartz glass and are welded together. This also ensures that the whole of the interior is hermetically sealed with respect to the environment, so that there is no possibility of contamination by the atmosphere of the environment. Furthermore, the injector is provided with an electrical heater, so that it and the gases passed through it can be preheated to a temperature of  greater than 500xc2x0 C.
At the tip of the injector, the gases which are fed to the injector are mixed with one another and are burned in a flame inside the balloon. Most injectors have been developed for an H2:O2 mixing ratio of approximately 1:0.65, in order to ensure complete oxidation of the hydrogen. In some applications, the opposite mixing ratio, namely 0.65:1, has proven particularly favourable, resulting in improved oxide uniformity. This means that significantly more oxygen is fed to the injector than would be required for purely stoichiometric combustion. However, this mixture is associated with a considerable increase in the temperature of the flame, with the result that the tip of the injector is xe2x80x9cburned offxe2x80x9d or devitrified. This leads to a considerable reduction in the service life of the injector. Furthermore, these phenomena may produce particles which cause further problems. Therefore, the entire combustion device becomes unusable and has to be repaired or rebuilt by exchanging the injector.
Exchanging the injector leads to an inevitable shutdown time for the process chamber and therefore reduces production rates.
To lengthen the service life of the injector, it has been attempted to modify its design, but usable results have not been achieved.
The same arrangement is also used to supply small quantities of oxygen (without any combustion process) to process chambers for pre-oxidation purposes by operating the mass flow controllers.
Therefore, the invention is based on the object of providing a process which is simple to implement and an arrangement for generating ultrapure steam which results in significantly lengthened injector service life.
The object on which the invention is based is achieved, in a process of the type described in the introduction, through the fact that oxygen and hydrogen are fed to the torch in a slightly superstoichiometric ratio, and that pure additional oxygen is admixed with the steam formed during the combustion, as it enters the process chamber, in order to generate steam having a considerable excess of oxygen.
This method, which is particularly simple to implement, leads to significant lengthening of the injector service life, while supplying the additional oxygen after combustion does not pose any problems. The invention makes it possible to select the quantity of additional oxygen and therefore the proportion of free oxygen in the steam independently, without the service life of the injector or of the balloon being adversely affected.
To avoid any cooling of the steam, with possible formation of condensate at the outlet of the balloon, the additional oxygen which is admixed with the steam may be preheated to a predetermined temperature. The preheating of the additional oxygen may be carried out directly by utilizing the thermal energy of the hydrogen flame.
According to a particularly advantageous configuration of the method, oxygen and hydrogen are fed to the torch in a ratio of 0.65:1, and additional oxygen is admixed with the steam formed during the oxidation process in the torch.
In a further configuration of the invention, the gases which are fed to the torch are preheated; the preheating temperature should be at least 100xc2x0 C.
Furthermore, the object on which the invention is based is achieved in an apparatus comprising a closed torch with feed lines for feeding hydrogen and oxygen and a discharge line leading to a process chamber. Each feed line is provided with a mass flow controller. An additional oxygen feed line is connected to the discharge line leading to the process chamber and is provided with a mass flow controller.
This allows the oxygen content in the steam which is generated to be set completely independently. The fact that the temperature of the hydrogen flame is not influenced in any way by the supply of the additional oxygen makes it possible to considerably lengthen the service life of the injector and therefore of the entire combustion device. This makes it possible to save considerable operating costs.
To allow sufficient preheating of the additional oxygen, the feed line for the additional oxygen may be provided with a heating device, and the heating device is arranged in the immediate vicinity of the connection of the tube to the discharge passage.
In a particular configuration of the invention, it is provided that the heating device comprises a pipe section of the feed line for the additional oxygen which at least partially surrounds the torch. As a result, the thermal energy of the flame can be used to preheat the additional oxygen.
To rule out any possibility of contamination, all the components of the combustion device, i.e. all the lines and the balloon, consist of quartz glass.